Finned tube heat exchangers are widely used in a variety of applications in the fields of refrigeration, air conditioning and the like. Such heat exchangers consist generally of a plurality of spaced parallel tubes through which a heat transfer fluid such as water, oil, air or a refrigerant is forced to flow while a second heat transfer fluid such as air is directed across the tubes. To improve heat transfer a plurality of fins comprising thin sheet metal plates are placed on the tubes. Each fin plate has a plurality of apertures through which the tubes pass generally at right angles to the fin, and a large number of the fins are arranged in parallel, closely spaced relationship along the tubes to form multiple paths for the air or other heat exchange fluid to flow across the fins and around the tubes. The tubes and plates are provided with a suitable mechanical and thermal bond, for example by expansion of the tubes after assembly of the fin plates, to provide good thermal conduction.
A great number of different fin designs for heat exchangers have been proposed in the prior art in the continual search for efficiency, compactness and manufacturing and operating economy. Since the fins are so important in the overall heat transfer of the heat exchanger, even a small increase in the heat transfer coefficient between the surface of the fin and the surrounding airstream or other heat transfer fluid can have an important beneficial effect on overall heat exchanger performance. Numerous fin designs have been proposed in the prior art using various techniques to increase the heat transfer coefficient. Fins such as those proposed in U.S. Pat. Nos. 3,397,741 and 3,438,433 improve efficiency by interrupting the fin plate with a number of short, flat louvers raised up from the plane of the fin, to cause numerous disruptions of the hydrodynamic boundry layers which form with increasing thickness along the fins and decrease heat transfer coefficient. From the standpoint of boundry layer disruption, the greatest improvement would be to have as large a number of very short louvers as possible. Unfortunately, such an approach leads to practical problems of weakness of the resulting thin sheet metal fin plate and this is very undesirable since it makes assembly of the heat exchanger difficult. U.S. Pat. No. 4,365,667 proposes solving this problem by adding stiffening corrugations to each of the louvers formed in the fin plate. The corrugations help stiffen each louver and also the entire fin plate. In addition, the corrugations in the louvers have the effect of turning the airstream. While turning the airstream does provide higher heat transfer than a straight air flow, it does not do it as efficiently, with regard to air pressure drop, as does repeatedly breaking the boundry layer with short, flat louvers. Thus the fin design proposed in the above mentioned patent solves the problem of increasing the heat transfer efficiency of the heat exchanger, but at the expense of increased air pressure drop. Since there is a cost involved in forcing the air across the fins of the heat exchanger, the air pressure drop is an important factor in the overall efficiency of a system using the finned tube heat exchanger.
Despite the progress which has been made in the field, there is still a need for a finned tube heat exchanger with increased heat transfer efficiency while maintaining the air pressure drop as low as possible. At the same time it is important to provide a certain degree of stiffness in the heat exchanger fins to simplify and speed up assembly and manufacturing procedures.